1. Field of the Invention
This invention relates to an electrically insulated pipe coupling and method for making the same and more particularly to a high pressure pipe coupling having components engaged in sealing relation by contact of sealing surfaces that increases the seal forces upon an increase in the pipeline pressure.
2. Description of the Prior Art
It is well known in the art to connect pipes of a pipeline, for example a main gas transmission line to a branch gas transmission, by an electrically insulated coupling. In order to cathodically protect the main or branch lines from the flow of current between the connected lines and to prevent rusting of the pipe, electrical insulation must be provided between the main line and the branch lines. The insertion of an annular insulator between the spaced apart pipes with an insulating sleeve surrounding the pipes and the insulator is a well known method of completing an insulated pipe coupling as disclosed in U.S. Pat. No. 4,011,652.
The coupling while serving to insulate one coupling member from another must also resist corrosive damage by the entrance of moisture into the coupling members. In most cases fluid transmission lines, such as a gas pipeline, are located underground where the lines are subjected to a corrosive environment. Unless the coupling members that connect the pipelines are connected in a manner to resist the entrance of moisture into the coupling members and the coupling threads, moisture will promote rust of the coupling members. Thread corrosion is generally the principal cause of leaks in underground, coupled pipelines.
It is the conventional practice to enclose the spaced apart electrically insulated coupling end portions of the pipeline connection in an insulating sleeve. The sleeve overlaps the annular insulator and the coupling end portions. The sleeve must be securely retained on the coupling end portions and a fluid tight seal maintained between the sleeve and the coupling end portions. One approach that has been suggested to sealingly engage the sleeve to the coupling end portions is the provision of annular beads on the coupling members anchored in sockets of the sleeve, as disclosed in U.S. Pat. No. 3,993,331.
A similar approach is the use of O-rings between the coupling members and the sleeve as disclosed in U.S. Pat. Nos. 3,871,687 and 4,147,381. U.S. Pat. No. 4,147,381 also discloses a sleeve having a plurality of ribs engaging complimentary grooves on the coupling members. It has also been suggested to adhesively bond the abutting surfaces of the sleeve and coupling members together.
A problem common to above methods of sealingly connecting the insulating sleeve to the coupling members is maintaining a seal around the annular insulator for a wide range of fluid pressures, both liquid and gas, through the coupling. There is also encountered the problem of preventing leakage through the insulating sleeve due to the porous nature of pressure carrying members constructed by the filament winding process. Under high pressures as encountered particularly in gas transmission lines, O-ring seals are subject to distortion and leakage of fluid around the annular insulator and between the sleeve and the coupling members. Also connections made by complimentary engaging surfaces of the sleeve and coupling members must be initially designed to withstand the maximum fluid pressure permitted in the pipeline. Consequently, the known pipeline couplings which possess a high insulating value and adequate strength to withstand high pressures are expensive to make and are complex to assemble.
Therefore, there is need in high pressure pipelines requiring an insulated coupling for connecting pipes, a coupling that is efficiently and inexpensively assembled to provide a fluid tight seal between the coupling members for a wide range of fluid pressures.